* Added final SWAP removal.

qopt_best_practices/transpilation/swap_cancellation_pass.py

@@ -0,0 +1,44 @@
+"""Pass to remove SWAP gates that are not needed."""
+
+from qiskit.dagcircuit import DAGOutNode, DAGCircuit
+from qiskit.transpiler import TransformationPass
+
+
+class SwapToFinalMapping(TransformationPass):
+    """Absorb any redundent SWAPs in the final layout.
+
+    This pass should be executed after a SWAPStrategy has been applied to a block
+    of commuting gates. It will remove any final redundent SWAP gates and absorb
+    them into the virtual layout. This effectively undoes any possibly redundent
+    SWAP gates that the SWAPStrategy may have inserted.
+    """
+
+    def run(self, dag: DAGCircuit):
+        """run the pass."""
+
+        qmap = self.property_set["virtual_permutation_layout"]
+
+        qreg = dag.qregs["q"]
+
+        # This will remove SWAP gates that are applied before anything else
+        # This remove is executed multiple times until there are no more SWAP
+        # gates left to remove. Note: a more inteligent DAG traversal could
+        # be implemented here.
+
+        done = False
+
+        while not done:
+            permuted = False
+            for node in dag.topological_op_nodes():
+                if node.op.name == "swap":
+                    successors = list(dag.successors(node))
+                    if len(successors) == 2:
+                        if all(isinstance(successors[idx], DAGOutNode) for idx in [0, 1]):
+                            bits = [qreg.index(qubit) for qubit in node.qargs]
+                            qmap[bits[0]], qmap[bits[1]] = qmap[bits[1]], qmap[bits[0]]
+                            dag.remove_op_node(node)
+                            permuted = True
+
+            done = not permuted
+
+        return dag